Temperature compensation in semiconductor circuits is well known. In both n and p type semiconductor devices as the temperature increases, a certain percentage of the more loosely held valence electrons produce free conduction electrons, and raising the temperature decreases the resistance of such semiconductor devices because of the increased number of free electron current carriers.
Diodes have been used in logic gate semiconductor circuits to track temperature variations. Band-gap circuits and FET summing circuits have been used to generate insensitive reference voltages over given temperature ranges. The use of diodes to track temperature variations require additional devices for each logic gate and hence, result in significant power increases. Furthermore, the tracking between the active devices, as transistors, FETs and so on, and the diodes is not sufficient to guarantee operation of a logic gate over a wide temperature range as, for example, the range required by military specifications. This temperature range is from -55.degree. C. to +125.degree. C. Diodes have been employed in temperature tracking circuits for many years. For example, a zener diode exhibits a breakdown voltage in a reverse bias state which increases as the diodes temperature increases. Similar measurements also show that the small voltage drop across a forward biased diode decreases as the temperature increases. These two opposing characteristics are combined and a temperature compensating unit can be formed to track temperature. Multiple diodes have also been employed to provide voltage references for logic gates and so on, which compensate temperature variations. Thus, the art of providing temperature compensation utilizing diodes is well-known. In any event, there is a serious problem with diode operation in regard to the temperature ranges discussed above. Diodes have also been employed together with transistors to provide circuits referred to as temperature compensated current sources. These operate in conjunction with differential amplifiers and other circuits but also exhibit the above-noted problems. One can employ thermistors and other temperature sensitive resistors, but these devices are non-linear devices and exhibit non-linear operation over wide temperature ranges. The band-gap and summing circuits as indicated above, generate a reference voltage during operation and do not improve the gate performance over wide temperature ranges.
A serious problem in employing modern technology in fabricating logic gates is that over the wide temperature range, there is an inadequate noise margin. Thus at elevated temperatures, the decreased noise margin limits the integration capability of gate arrays. In other words, for the high end of the temperature range in military applications, one cannot provide integrated circuit logic gates which will operate reliably. This is because of the fact that the gate performance decreases as the temperature increases and based on this factor, noise and other spurious signals will cause the gates to falsely operate, thereby creating circuits which are unreliable in the presence of noise.
It is therefore an object of the present invention to provide a temperature compensation circuit which will allow semiconductor logic gates to function reliably with adequate noise margin over a wide temperature range.
The circuit to be described further eliminates the need for any additional devices and eliminates the need for modification of the gate circuit itself.
The term "gate" includes logic integrated circuit gates which are fabricated on integrated circuit substrates and which include diode and transistor configurations as is well known. In logic systems, the three basic building blocks or logic gates include the inverter, the OR gate, and the AND gate. From these three components, most logic blocks can be formed, as for example, counters, registers and so on.
Thus, the compensation circuit to be described allows the use of logic gates which are fabricated utilizing integrated circuit techniques and formed from gallium arsenide (GaAs) and silicon LSI circuits. The gates provide adequate noise margins at the elevated temperatures and operate over the wide temperature range due to the temperature compensation apparatus according to this invention.